Metal Olefin Research Articles

Late Transition Metal Olefin Polymerization Catalysts Derived from 8-Arylnaphthylamines

Late transition metal catalysts represent a significant class of olefin polymerization catalysts that have played an essential role in advancing the polyolefin industry owing to their highly tunable ligands and low oxophilicity. A key feature for the design of late transition metal catalysts lies in the steric bulk of the o-aryl substituents. Bulky 8-arylnaphthylamines have emerged as a promising aniline candidate for conducting high-performance catalysts by introducing axially steric hindrance around the metal center. This review focuses on late transition metal (Ni, Pd, Fe) catalysts derived from 8-arylnaphthylamines, surveying their synthesis, structural features, and catalytic applications in olefin (co)polymerizations. Additionally, the relationship between catalyst structure and catalytic performance is discussed, highlighting how these unique ligand systems influence polymerization activity, molecular weight, and polymer branching.

Hybrid cycloolefin ligands for palladium–olefin cooperative catalysis

The engineering of molecular catalysts in homogeneous catalysis enables new catalytic modes, which leads to efficient synthetic transformations. In this regard, the design of ligands for transition metal catalysis has played a major role. In transition metal catalysed reactions, olefins can serve as steering ligands by tuning the electronic and steric nature of the metal centre. Here we report unstrained olefin ligands that bear a P or S coordination site for use in the Pd-catalysed Catellani reaction. This olefin ligand shows a covalent catalytic function and enables an efficient ipso,ortho-difunctionalization of iodoarenes. Mechanistic analysis reveals a reversible covalent bonding between the substrate and the cycloolefin unit of the ligand, which forms key organopalladium intermediates to enable a new reactivity. Our results demonstrate a design concept that employs hybrid cycloolefin ligands as a covalent catalytic module, which opens up possibilities for further cooperative catalysis with other transition metal–olefin hybrids. Taking inspiration from palladium–norbornene cooperative catalysis, Catellani-type reactions are now performed using a hybrid olefin ligand with a P or S coordination site. This olefin ligand enables efficient ipso,ortho-difunctionalization of iodoarenes. Mechanistic studies show the formation of organopalladium intermediates that comprise both the substrate and the hybrid olefin ligand.

Identifying the Imperative Role of Metal-Olefin Interactions in Catalytic C-O Reductive Elimination from Nickel(II).

We present a series of experimental and computational mechanistic investigations of an unusually facile example of Ni-catalyzed C-O bond formation. Our method, originally reported in 2016, involves the formation of cyclic enol ethers from vinyl iodides bearing pendant alcohol groups. Our findings suggest that the observed reactivity arises from the coordination of the olefin in the vinyl iodide starting material and the enol ether product with Ni(0) intermediates. Density functional theory calculations reveal a plausible catalytic mechanism involving a Ni(II)/Ni(0) redox cycle featuring two-electron C-I oxidative addition and C-O reductive elimination steps. The direct formation of a η 2-enol ether Ni(0) complex from a key Ni(II) alkoxide intermediate dramatically alters the free energy (ΔG) for the vinyl C-O reductive elimination step relative to other examples of C-O reductive elimination at Ni(II). Furthermore, efficient σ-π mixing in the course of vinyl C-O reductive elimination leads to lower computed kinetic barriers (ΔG ‡) relative to those of aryl C-O reductive elimination. The conclusions drawn from these computational models are supported by synthetic organometallic experiments, whereby a vinyl-Ni(II) iodide intermediate was isolated, characterized, and proved to yield enol ether, following exposure to triethylamine. We conducted further experiments and computations, which indicated that the two-electron oxidative addition of vinyl iodides by Ni(0) depends on the formation of an η 2-vinyl iodide precomplex, based on the observation of one-electron activation of the same vinyl iodide in the presence of sterically encumbering ligands (e.g., tricyclohexylphosphine).

Revealing the surface atomic arrangement of noble metal alkane dehydrogenation catalysts by a stepwise reduction-oxidation approach

Revealing the surface atomic arrangement of noble metal alkane dehydrogenation catalysts by a stepwise reduction-oxidation approach

Unsupported Mg-Alkene Bonding.

The first intermolecular early main group metal–alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base‐free cations (MeBDI)Mg+ and (tBuBDI)Mg+ with B(C6F5)4 − counterions (MeBDI=CH[C(CH3)N(DIPP)]2, tBuBDI=CH[C(tBu)N(DIPP)]2, DIPP=2,6‐diisopropylphenyl). Coordination complexes with various mono‐ and bis‐alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3‐divinyltetramethyldisiloxane, 1,5‐cyclooctadiene, cyclooctene, 1,3,5‐cycloheptatriene, 2,3‐dimethylbuta‐1,3‐diene, and 2‐ethyl‐1‐butene. In all cases, asymmetric Mg–alkene bonding with a short and a long Mg−C bond is observed. This asymmetry is most extreme for Mg–(H2C=CEt2) bonding. In bromobenzene solution, the Mg–alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg–alkene bonds are mainly electrostatic and should be described as Mg2+ ion‐induced dipole interactions.

Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized.

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal-alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5 H5 )Ru(NCMe)3 ]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.

Metal Olefin Complexes: Revisiting theDewar−Chatt−DuncansonModel and Deriving Reactivity Patterns from Carbon‐13 NMR Chemical Shift

AbstractMetal olefin complexes that are ubiquitous intermediates in catalysis are investigated by a detailed analysis of their13C‐NMR chemical shift tensors. This analysis allows evidencing specific electronic features, namely the olefin‐to‐metalσ‐donation and the metal‐to‐olefinπ‐backdonation as proposed in theDewar−Chatt−Duncansonmodel. Apart from these interactions, the chemical shift tensor analysis reveals an additional ligand‐to‐metalπ‐donation of the olefinσ(C=C) orbital in systems with suitably oriented vacant d‐orbitals. This interaction which is not accounted for in theDewar−Chatt−Duncansonmodel explains the reactivity of this type of metal olefin complexes towards oxidative cyclization (olefin insertion) and protonolysis.

Recent advances in thermally robust, late transition metal‐catalyzed olefin polymerization

AbstractOlefin polymerization catalysts employing late transition metals, such as Ni, Pd, Fe and Co, have been heavily investigated since their discovery in the mid‐1990s. Despite the advantages and appeal these catalysts have displayed in academic research, their implementation in industry has remained restricted due to a variety of limitations and drawbacks. One specific limitation is the relative thermal instability of most late transition metal olefin polymerization catalysts at the temperatures commonly utilized for industrial polymerizations. This review focuses on the development of Ni‐, Pd‐, Fe‐ and Co‐based olefin polymerization catalysts that display increased thermal stability at super‐ambient reaction temperatures. These advances in thermal stability have been realized using a variety of ligand modifications, and their polymerization activities and thermal stability are summarized herein. Lastly, a brief outlook on the future of thermally stable, late transition metal olefin polymerization catalysts is provided. © 2018 Society of Chemical Industry

Research progress of pre-transition metal olefin polymerization catalyst for salicylaldehyde imine

Salicylaldehyde imine transition metal catalyst is a kind of olefin polymerization catalyst that is widely used in the coordination of salicylaldehyde imine ligand and pre-transition metal. Salicylaldehyde imine ligands have the characteristic of easily inserting different substituents via organic synthesis. Therefore, the regulation of the polymerization activity, polymerization product, and product distribution can be achieved by changing the steric hindrance effect, the electronic effect, and the number of metal active sites near the catalytic active center. The development status of the transition metal catalyst of salicylaldehyde imide was summarized in this paper. The influence of the ligand structure of the salicylaldehyde imide transition metal catalyst on the catalytic performance, which involved the high selectivity of ethylene trimerization, ethylene/α-olefin, polar monomer copolymerization, ethylene polymerization production, ultra-high molecular weight polyethylene, and many other areas of olefin polymerization, was elaborated, providing references for further study and industrial applications of this catalyst.

Research Progress of Pre - transition Metal Olefin Polymerization Catalyst for Salicylaldehyde Imine

Salicylaldehyde imine transition metal catalyst is a kind of olefin polymerization catalyst which is widely used in the coordination of salicylaldehyde imine ligand and pre-transition metal. Salicylaldehyde imine ligands have the characteristics of easily insert different substituents via organic synthesize. Therefore, the regulation of the polymerization activity, polymerization product and product distribution can be achieved by changing the steric hindrance effect, electronic effect and the number of metal active sites which near the catalytic active center. The development status of the transition metal catalyst of salicylaldehyde imide was summarized in this paper. The influence of the ligand structure of salicylaldehyde imide transition metal catalyst on the catalytic performance which involved the high selectivity ethylene trimerization, ethylene / α-olefin, / Polar monomer copolymerization, ethylene polymerization production of ultra-high molecular weight polyethylene and many other areas of olefin polymerization was elaborated and providing references for further study and industrial applications of this catalyst.

Cyclometallated Platinum(II) Complexes with a Phenylpropene‐Derived π/σ‐Chelator and N‐Heterocyclic Carbenes

A series of heteroleptic platinum(II) complexes of the general formula [PtX(iPrEug)(NHC)] (2–7) bearing the organometallic π/σ‐chelator iPrEug (isopropyl eugenoxyacetate), with varying X (halido) and NHC (N‐heterocyclic carbene) ligands derived from imidazole, benzimidazole, and triazole, were prepared and fully characterized by elemental analyses, ESI mass spectrometry, and IR and NMR spectroscopy. Complexes 3 and 5–7 were also characterized by single‐crystal X‐ray diffraction. These air‐stable complexes are rare examples of platinum(II) compounds containing three different types of carbon donors, that is, aryl, carbene, and olefin. Unsymmetrical NHCs led to rotameric pairs of 4 and 5 exhibiting different NMR spectroscopic features. Spectroscopic studies also revealed that the introduction of a NHC leads to a weakening of the metal–olefin bond and strengthening of the metal–aryl bond. On the other hand, backbone and thus electronic variations among the NHCs show little influence on the bonding of the iPrEug chelator.

Low temperature in situ synthesis and the formation mechanism of various carbon-encapsulated nanocrystals by the electrophilic oxidation of metallocene complexes

The core–shell nanostructures have the advantages of combining distinctive properties of varied materials and improved properties over their single-component counterparts. Synthesis approaches for this class of nanostructures have been intensively explored, generally involving multiple steps. Here, a general and convenient strategy is developed for one-step in situ synthesis of various carbon-encapsulated nanocrystals with a core–shell structure via a solid-state reaction of metallocene complexes with (NH4)2S2O8 in an autoclave at 200 °C. A variety of near-spherical and equiaxed nanocrystals with a small median size ranging from 6.5 to 50.6 nm are prepared as inner cores, including Fe7S8, Ni3S4 and NiS, CoS, TiO2, TiO2 and S8, ZrO2, (NH4)3V(SO4)3 and VO2, Fe7S8 and Fe3O4, MoS2 and MoO2. The worm-like carbon shell provides exclusive room for hundreds of nanocrystals separated from each other, preventing nanocrystal aggregation. The synergistic effect of ammonium and a strong oxidizing anion on the electrophilic oxidation of metallocene complexes containing a metal–ligand π bond contributes to the carbon formation at low temperature. It is considered that the cyclopentadienyl ligand in a metallocene complex will decompose into highly reactive straight chain olefinic pieces and the metal–olefin π interaction enables an ordered self-assembly of olefinic pieces on nanocrystals to partially form graphitizable carbon and a core–shell structure. The high capacity, good cycling behavior and rate capability of Fe7S8@C and Ni3S4 and NiS@C electrodes are attributed to the good protection and electrical conductivity of the carbon shell.

Observation of Cationic Transition Metal-Alkane Complexes with Moderate Stability in Hydrofluorocarbon Solution.

In seeking to create more-stable transition metal-alkane complexes, we generated cationic alkane complexes of the type [(HEB)Re(CO)2(alkane)][Al(OR(f))4] (HEB = η(6)-hexaethylbenzene; alkane = cyclopentane (16) or pentane (17-19); OR(f) = perfluoro-tert-butoxy) via photolysis of the precursor complex [(HEB)Re(CO)3][Al(OR(f))4] (15) in the presence of the added alkane. The alkane complexes were generated in a hydrofluorocarbon (HFC) solvent, most often CF3CH2CF3, which is capable of simultaneously dissolving the ionic complex 15 and a small amount of alkane at low temperature (183 K). Use of the HFC solvent in tandem with the highly fluorinated, solubilizing, weakly coordinating [Al(OR(f))4](-) anion overcomes the technical difficulty of combining ionic species with alkanes in solution without the solvent molecules rapidly displacing the bound alkane ligand, as the alkanes bind in preference to the HFCs to the organometallic fragments employed in this study. The [(HEB)Re(CO)2(alkane)](+) complexes are more long-lived than the corresponding neutral alkane complexes [(HEB)W(CO)2(alkane)] and [CpRe(CO)2(alkane)] (Cp = η(5)-cyclopentadienyl), with samples of [CpRe(CO)2(cyclopentane)] decaying significantly more rapidly than [(HEB)Re(CO)2(alkane)](+) when present in the same solution. Intramolecular exchange of the methylene group bound to the metal within the cyclopentane ligand in 16 was observed at 212 K, with the 1,2 shifts appearing to be faster than 1,3 shifts.

Electronic Structure of Ti3+–Ethylene Complexes in Microporous Aluminophosphate Materials. A Combined EPR and DFT Study Elucidating the Role of SOMO Orbitals in Metal–Olefin π Complexes

The interaction of tetrahedrally coordinated Ti3+ ions generated in the framework of TiAlPO-5 microporous materials with 12,13C2H4 leads to the formation of side-on η2 {Ti3+—C2H4} complexes with a unique 5-fold coordination of titanium, supported by four oxygen donor ligands of the framework. The detailed electronic and magnetic structure of this adduct is obtained by the combination of advanced EPR techniques (HYSCORE and SMART-HYSCORE) in conjunction with periodic and cluster model DFT calculations. The binding of C2H4 results from the σ overlap of low lying C2H4 filled π orbitals with the 3dz2 empty orbital of titanium, enhanced by a small contribution due the π overlap between the semioccupied 3dyz orbital of titanium and the empty π* orbital of ethylene. The spin density repartition over the ethylene molecule, obtained experimentally, allows probing directly the entity of the metal-to-substrate π*-back-donation, highlighting an asymmetry in the spin density delocalization. This interesting feature is supported by parallel theoretical calculations, which cast the role of the oxygen donor ligands in driving this bonding asymmetry. As a consequence, the interesting structural feature of potential and actual inequality in the electronic spin states (α,β) on the two ethylene carbon atoms of the π coordinated ethylene molecule is produced. The underlying electronic effects associated with the π coordination of ethylene to an early transition metal in paramagnetic state are thus revealed with an unprecedented accuracy for the first time.

Discovery and Development of Pyridine-bis(imine) and Related Catalysts for Olefin Polymerization and Oligomerization.

For over 40 years following the polyolefin catalyst discoveries of Hogan and Banks (Phillips) and Ziegler (Max Planck Institute), chemists traversed the periodic table searching for new transition metal and lanthanide-based olefin polymerization systems. Remarkably, none of these "hits" employed iron, that is, until three groups independently reported iron catalysts for olefin polymerization in the late 1990's. The history surrounding the discovery of these catalysts was only the beginning of their uniqueness, as the ensuing years have proven these systems remarkable in several regards. Of primary importance are the pyridine-bis(imine) ligands (herein referred to as PDI), which produced iron catalysts that are among the world's most active for ethylene polymerization, demonstrated "staying power" despite over 15 years of ligand improvement efforts, and generated highly active polymerization systems with cobalt, chromium, and vanadium. Although many ligands have been employed in iron-catalyzed polymerization, the PDI family has thus far provided the most information about iron's capabilities and tendencies. For example, iron systems tend to be highly selective for ethylene over higher olefins, making them strong candidates for producing highly crystalline polyethylene, or highly linear α-olefins. Iron PDI polymerizes propylene with 2,1-regiochemistry via a predominantly isotactic, chain end control mechanism. Because the first insertion proceeds via 1,2-regiochemistry, iron (and cobalt) PDI systems can be tailored to make highly linear dimers of α-olefins by "head-to-head" coupling, resulting from a switch in regiochemistry after the first insertion. Finally, PDI ligands, while not being surpassed in activity, have inspired the development of related ligand families and complexes, such as pendant donor diimines (PDD), which are also highly efficient at producing linear α-olefins. This Account will detail a variety of oligomerization and polymerization results achieved with PDI and PDD catalysts. Our studies on ligand modification are discussed, but numerous ligands have been synthesized by others. Computational approaches, identification of catalyst active sites, noninnocent ligand studies, commercialization efforts, and other outstanding research are only briefly mentioned, at most. The reader is directed to review articles where appropriate, in order to address the cursory treatment of these areas.

Fluorescence of retinoic acid in the presence of metal salts

Retinoic acid (vitamin A acid) in acetonitrile shows a very weak fluorescence which originates from the lowest-energy ππ* singlet of the polyene chain. In the presence of several metal salts, Hg(acetate)2, Ag(CF3COO) and ZnCl2, this fluorescence undergoes a slight spectral shift and becomes much more intense. The reason is not quite clear, but different effects may cause this fluorescence enhancement, e.g. energy increase of the non-emissive nπ* state by coordination at the carboxylic group, metal–olefin coordination and an increase of rigidity by organized structures.

Early Main Group Metal Catalysis: How Important is the Metal?

AbstractOrganocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca2+. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph2N−][Me4N+] and [Ph3C−][Me4N+]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of ArNCO and RNCNR (R=alkyl) by Ph2NH. For the intramolecular hydroamination of unactivated CC bonds in H2CCHCH2CPh2CH2NH2 the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

Characterisation of alkane σ-complexes.

Alkane σ-complexes have evolved from a curious phenomenon to an intermediate of intense interest, fuelling research into the area. Over the last fifteen years, metal alkane complex characterisation has evolved to incorporate reports employing UV/Vis, IR and NMR spectroscopy, and X-ray and neutron diffractometry. Previously, due to the sparse geometric characterisation of alkane σ-complexes, assumptions regarding bonding geometries and selectivities were made by comparison to related σ-complexes, or by analysis of C-H activation products. This minireview assembles relevant literature that illuminates the metrics of alkane-metal bonding, and critically analyses the binding mode, selectivity and stability of alkane complexes.

A cyclo-octa-trienone complex of diiron hexa-carbon-yl.

In the title compound, [μ-(2,6,7-η:3,4,5-η)-cycloocta-2,4,6-trienone]bis­(tri­carbonyl­iron)(Fe—Fe), [Fe2(C8H8O)(CO)6], the diiron hexa­carbonyl moiety has a sawhorse arrangement, with the OC—Fe—Fe—CO fragment forming the horizontal bar of the horse, and the other four carbonyl groups the legs. The Fe—Fe distance is 2.795 (2) Å. Each Fe atom is also bonded to three C atoms of the cyclo­octa­trienone ring. One Fe atom forms a σ-bond with one ring C atom, with Fe—C = 2.109 (2) Å, and also a metal–olefin π-bond with two C atoms on the other side of the ring, with Fe—C distances of 2.238 (2) and 2.236 (3) Å. The second Fe atom forms a η3-allyl bond with three other ring atoms, with Fe—C bond lengths of 2.158 (2), 2.062 (2), and 2.123 (3) Å. Counting the π- and π-allyl inter­actions as one bond, the coordinations of the Fe atoms can, respectively, be approximated as octa­hedral and trigonal bipyramidal.

Evaluation of Reactive Absorption of Ethylene over Ethane Using Copper Metal Solution through AspenPlus Reactive Absorption–Regeneration Model

Traditionally, olefin/paraffin separations are carried out using cryogenic distillation. The close relative volatility, subambient temperatures and high pressures required in cryogenic distillation make the cryogenic distillation an energy-intensive and capital-intensive separation scheme. Significant energy savings could potentially be realized if a nondistillation separation could prefractionate the olefins from the paraffins for more optimum placement in the downstream distillation unit or eventually completely replace the need for distillation. Reactive absorption with metal species carried in a solvent is one of the techniques that use a transition metal to selectively bind the olefin through formation of an olefin–metal π-complex. An experimental investigation into the selective absorption of ethylene over ethane using a novel copper metal solution containing aniline–CuCl–NMP was reported in the literature [Reine, Travis; Olefin/Paraffin Separation by Reactive Absorption. PhD Thesis, University of Texas at Austin, 2004]. The present work makes use of this work and models this reactive absorption–regeneration in AspenPlus for the selective absorption of ethylene over ethane using aniline–CuCl–NMP solution. The Aspen model predicted the experimental results for ethylene recovery and selectivity at varied liquid-to-gas ratios in reasonable agreement with the experimental results from the literature. The model can be used for a scale-up purpose and can also be extended for selective absorption of higher olefin/paraffin systems. It is also possible to use the simulation approach presented here to guide the research in this area, potentially reducing the energy consumption for the production of these key products.